迭代器中的转换错误，超过unordered_map的向量

Question

我有一个简单的程序，我想在无序地图的向量上迭代。我是STL的新手，不知道为什么转换存在问题。代码如下所示，

#include<iostream>
#include<vector>
#include<unordered_map>

std::unordered_map<uintptr_t, uintptr_t> umap1{ { 1, 2 }, { 3, 4 }, { 5, 6 } };
std::unordered_map<uintptr_t, uintptr_t> umap2{ { 7, 8 }, { 9, 10 }, { 11, 12 } };
std::unordered_map<uintptr_t, uintptr_t> umap3{ { 13, 14 }, { 15, 16 }, { 17, 18 } };

std::vector<std::unordered_map<uintptr_t, uintptr_t>> crd{ umap1, umap2, umap3 };

constexpr uint8_t ZERO = 0;

std::vector<std::unordered_map<uintptr_t, uintptr_t>>::iterator begin()
{
    return crd[ZERO].begin();
}

std::vector<std::unordered_map<uintptr_t, uintptr_t>>::iterator end()
{
    return crd[ZERO].end();
}

int main()
{
    for (auto it = begin(); it != end(); ++it)
    {
        for (auto it1 = it->begin(); it1 != it->end(); ++it)
        {
            std::cout << it1->first << " " << it1->second << std::endl;
        }
    }
    return 0;
}

这是错误，

mytest.cpp(15): error C2440: 'return': cannot convert from 'std::_List_iterator<std::_List_val<std::_List_simple_types<_Ty>>>' to 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>'
        with
        [
            _Ty=std::pair<const uintptr_t,uintptr_t>
        ]
        and
        [
            _Ty=std::unordered_map<uintptr_t,uintptr_t,std::hash<uintptr_t>,std::equal_to<uintptr_t>,std::allocator<std::pair<const uintptr_t,uintptr_t>>>
        ]
mytest.cpp(15): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
mytest.cpp(20): error C2440: 'return': cannot convert from 'std::_List_iterator<std::_List_val<std::_List_simple_types<_Ty>>>' to 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<_Ty>>>'
        with
        [
            _Ty=std::pair<const uintptr_t,uintptr_t>
        ]
        and
        [
            _Ty=std::unordered_map<uintptr_t,uintptr_t,std::hash<uintptr_t>,std::equal_to<uintptr_t>,std::allocator<std::pair<const uintptr_t,uintptr_t>>>
        ]
mytest.cpp(20): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

任何帮助都将不胜感激！提前致谢。